Nanotechnology Spotlight

(Nanowerk Spotlight) Nature often provides the blueprint when researchers are developing new technologies. Just think of George de Mestral and how, back in 1948, his observation of the structure of cockleburr seeds led to the development of Velcro®. In a similar vein, observations made on the Salvinia fern as well as the Notonecta glauca bug now have led researchers to develop a nanofur structure that significantly reduces fluid drag.

For instance, consider the big amount of fuel used by the 90,000 ocean-going cargo ships that roam the seas (international shipping uses about 300 million metric tons of fuel and it is estimated it is responsible for 3.5% to 4% of all climate change emissions). Most of the energy in shipping is used to overcome surface friction. Therefore an effective way to reduce frictional drag underwater could significantly reduce marine fuel consumption, thus making the shipping industry more efficient and environmental friendly.

Additionally, in most applications which involve moving liquids through pipes and tubes of different sizes, a lot of the energy is used to overcome the drag the fluid experiences moving over the sidewalls. Here as well, a drag reducing coating could compensate this effect and increase efficiency in these areas.

Both the fern and the insect have surfaces covered by high density hairs which allow them to keep an air layer under water. This enables the Notonecta glauca bug to move nimbly and swiftly through the water by reducing the drag on its surface.

Scientists at the Institute for Microstructure Technology (IMT), Karlsruhe Institute of Technology, have developed a very inexpensive, highly scalable method to produce a superhydrophobic, air retaining biomimetic surface – a 'nanofur' – which shows not only a high long-term stability but also a high resistance against additional applied pressure. These properties enable the surface to significantly reduce the frictional drag experienced by fluids over a wide range of flow rates.

"The most exciting result of our research is that the produced nanofur can not only hold an air film under water for more than 31 days (as detailed in previous work), but also shows a high stability regarding additional hydrostatic pressure," Dr. Maryna Kavalenka, a postdoc at IMT the first author of the paper, tells Nanowerk. "Most other surfaces show a much shorter life time of the retained air film or withstand only lower hydrostatic pressure."

It is precisely that stable air film on the nanofur surface that enables it to significantly reduce fluid drag. In experiments that tested the performance of nanofur coated surfaces, the measured pressure drop across the channels lined with nanofur is approximately 50% lower than in the channels lined with unstructured polymer – lower pressure
drop for the nanofur indicates the reduction in fluid drag by the material, resulting from the air layer retained on its nano- and microstructured surface.

The nanofur is produced using a hot pulling technique, which was developed at the IMT. In contrast to other methods used to produce superhydrophobic, air retaining and drag reducing surfaces, the hot pulling method is very low-cost, because it uses sandblasted steel plates as molds, an inexpensive fabrication procedure and material.

In addition, the process is highly scalable and uses no additional chemicals which could be toxic or harmful, thus making the nanofur easy to handle.

Nanofur fabricated on the polymer surface is inspired by hair-covered surfaces of plants and insects. (A) SEM images of the nanofur fabricated from polycarbonate. The tips of the hairs are less than 200 nm in diameter. (B) Cross-sectional SEM image of the nanofur reveals that hairs are tens of microns long. (C) Schematic of the hot pulling technique with a heated sandblasting steel plate used for nanofur fabrication. (D) Photograph of the water droplet on the superhydrophobic nanofur surface with contact angle θ ∼ 153°. (Reprinted with permission by American Chemical Society) (click on image to enlarge)

"Novel in our study is that we used a simple method to investigate the dynamic collapse of the air retention of the nanofur, by analyzing pictures taken with a camera," explains Kavalenka. "Other studies investigated periodic or highly symmetric structures for air retention and drag reduction, which therefore mostly had one specific critical pressure at which the collapse of the air/water interface happened. The nanofur, on the other hand, is covered by a layer of randomly distributed high aspect ratio nano- and microhairs interspersed with microcavities, analogous to the natural model air-retaining surfaces."

Because of its highly irregular shape and surface, the nanofur does not have one critical pressure at which the water-air-interface collapses. Because of this the scientists investigated the dynamic behavior of the air-water-interface in respect to the applied hydrostatic pressure.

Even though the air film held under water by the nanofur shows great long time stability and a high resistance against hydraulic pressure, the team is now investigating if it is possible to increase the resistance of the air-retaining layer against hydrostatic pressure. This could further improve the frictional drag reduction of the nanofur under water and open the door to a wide range of applications.

"The future direction of the drag reduction and air retention research of our group is mainly focused on increasing the long term stability, as well as the stability against additional hydrostatic pressure," concludes Kavalenka. "This, combined with research into the drag reduction for higher flow rates as they occur on container ships and other types of marine vessels, is going to be very interesting. This field poses the special challenge of shear forces which might lead to a loss of the air film. Combining this with harsh environmental conditions for the drag reducing surfaces leads to many challenges that we'll have to overcome in fabricating these materials."